<%@ WebHandler Language="C#" Class="HttpCombiner" %>

using System;
using System.Net;
using System.IO;
using System.IO.Compression;
using System.Text;
using System.Configuration;
using System.Web;

public class HttpCombiner : IHttpHandler {

    private const bool DO_GZIP = true;
    private readonly static TimeSpan CACHE_DURATION = TimeSpan.FromDays(30);
    
    public void ProcessRequest (HttpContext context) {
        
        HttpRequest request = context.Request;
        
        // Read setName, contentType and version. All are required. They are
        // used as cache key
        string setName = request["s"] ?? string.Empty;
        string contentType = request["t"] ?? string.Empty;
        string version = request["v"] ?? string.Empty;

        // Decide if browser supports compressed response
        bool isCompressed = DO_GZIP && this.CanGZip(context.Request);

        // Response is written as UTF8 encoding. If you are using languages like
        // Arabic, you should change this to proper encoding 
        UTF8Encoding encoding = new UTF8Encoding(false);

        // If the set has already been cached, write the response directly from
        // cache. Otherwise generate the response and cache it
        if (!this.WriteFromCache(context, setName, version, isCompressed, contentType))
        {
            using (MemoryStream memoryStream = new MemoryStream(5000))
            {
                // Decide regular stream or GZipStream based on whether the response
                // can be cached or not
                using (Stream writer = isCompressed ?
                    (Stream)(new GZipStream(memoryStream, CompressionMode.Compress)) :
                    memoryStream)
                {

                    // Load the files defined in <appSettings> and process each file
                    string setDefinition = 
                        System.Configuration.ConfigurationManager.AppSettings[setName] ?? "";
                    string[] fileNames = setDefinition.Split(new char[] { ',' }, 
                        StringSplitOptions.RemoveEmptyEntries);
                    
                    foreach (string fileName in fileNames)
                    {
                        byte[] fileBytes = this.GetFileBytes(context, fileName.Trim(), encoding);
                        writer.Write(fileBytes, 0, fileBytes.Length);
                    }

                    writer.Close();
                }

                // Cache the combined response so that it can be directly written
                // in subsequent calls 
                byte[] responseBytes = memoryStream.ToArray();
                context.Cache.Insert(GetCacheKey(setName, version, isCompressed),
                    responseBytes, null, System.Web.Caching.Cache.NoAbsoluteExpiration,
                    CACHE_DURATION);

                // Generate the response
                this.WriteBytes(responseBytes, context, isCompressed, contentType);
            }
        }
    }

    private byte[] GetFileBytes(HttpContext context, string virtualPath, Encoding encoding)
    {
        if (virtualPath.StartsWith("http://", StringComparison.InvariantCultureIgnoreCase))
        {
            using (WebClient client = new WebClient())
            {
                return client.DownloadData(virtualPath);
            }
        }
        else
        {
            string physicalPath = context.Server.MapPath(virtualPath);
            byte[] bytes = File.ReadAllBytes(physicalPath);
            // TODO: Convert unicode files to specified encoding. For now, assuming
            // files are either ASCII or UTF8
            return bytes;
        }
    }

    private bool WriteFromCache(HttpContext context, string setName, string version, 
        bool isCompressed, string contentType)
    {
        byte[] responseBytes = context.Cache[GetCacheKey(setName, version, isCompressed)] as byte[];

        if (null == responseBytes || 0 == responseBytes.Length) return false;

        this.WriteBytes(responseBytes, context, isCompressed, contentType);
        return true;
    }

    private void WriteBytes(byte[] bytes, HttpContext context, 
        bool isCompressed, string contentType)
    {
        HttpResponse response = context.Response;

        response.AppendHeader("Content-Length", bytes.Length.ToString());
        response.ContentType = contentType;
        if (isCompressed)
            response.AppendHeader("Content-Encoding", "gzip");

        context.Response.Cache.SetCacheability(HttpCacheability.Public);
        context.Response.Cache.SetExpires(DateTime.Now.Add(CACHE_DURATION));
        context.Response.Cache.SetMaxAge(CACHE_DURATION);
        context.Response.Cache.AppendCacheExtension("must-revalidate, proxy-revalidate");

        response.OutputStream.Write(bytes, 0, bytes.Length);
        response.Flush();
    }

    private bool CanGZip(HttpRequest request)
    {
        string acceptEncoding = request.Headers["Accept-Encoding"];
        if (!string.IsNullOrEmpty(acceptEncoding) &&
             (acceptEncoding.Contains("gzip") || acceptEncoding.Contains("deflate")))
            return true;
        return false;
    }

    private string GetCacheKey(string setName, string version, bool isCompressed)
    {
        return "HttpCombiner." + setName + "." + version + "." + isCompressed;
    }

    public bool IsReusable
    {
        get
        {
            return true;
        }
    }

}